There is interest in welding of aluminum-to-aluminum sheet assemblies and steel-to-steel sheet assemblies on the same manufacturing line. A prototypical application might be a single automotive vehicle door fabrication line used to process all-aluminum doors and all-steel doors in subsequent batches. Although one approach to that need would be to maintain substantial redundant systems in parallel, i.e. steel welding robots/equipment alongside aluminum joining robots/equipment, another approach would be to use one set of robots and change from steel welding equipment to aluminum joining equipment, i.e., clinch, rivet, or weld guns, and a third approach would be to develop equipment, especially welding electrodes, which could weld either material with only modest transitioning requirements.
In this work it is assumed that medium-frequency direct current (MFDC) weld controls and transformers could be installed with capabilities to weld either aluminum sheet assemblies or steel sheet assemblies with only weld schedule variations (including weld force) required between the two processes. However, different spot welding electrodes are currently used for aluminum and steel welding, respectively, and it is not presently possible to alternately weld aluminum sheet assemblies and steel sheet assemblies using the same welding electrodes on an existing production line.
It is one object of this invention to provide a single electrical resistance welding electrode design that is suitable for spot welding of two or more steel alloy sheets as they are presented for welding and for spot welding of two or more aluminum alloy sheets when they are to be welded. Further, that electrode geometry can be maintained by a single electrode face dressing system that provides longer life for both the electrodes and the electrode face dressing tools.
There is also a need for an electrode design and dressing process that prepares electrodes for welding either light metals such as aluminum alloy and magnesium alloy sheets or steel sheets. The high electrical and thermal conductivity of aluminum and magnesium alloys in combination with the insulating nature of the naturally-formed surface oxide of these materials makes them difficult to resistance spot weld using conventional spot welding practice. The spot welding process is sensitive to a large number of variables beyond the normal welding parameters of electrode configuration, electrode force, weld time, and weld current. These other variables include sheet surface oxidation, sheet surface cleanliness, sheet surface topography as well as process variations such as alignment of the electrodes to the sheet, location of electrodes relative to the sheet edge and part radius, metal fit up, gun stiffness, alignment of electrodes on the gun, electrode surface roughness, and wear of the electrode surface.
Further, for a typical automotive closure such as a side door, the material thickness of the aluminum or magnesium component for comparable panels (e.g., a door inner panel) is larger. For a comparably sized door, an aluminum inner door panel would likely be about 25%-75% thicker than a steel inner door panel. Similarly, an aluminum outer door panel would likely be about 25%-75% thicker than a steel outer door panel. Also, aluminum door reinforcements would also be 25%-75% thicker. Thus, a single electrode geometry intended to weld both aluminum and steel components on the same assembly line must be capable of welding stackups of aluminum that are substantially thicker than the comparable stackups of steel.
Dressing processes that machine the surfaces of spot welding electrodes have been used previously for spot welding steel sheet for closures and structures, and aluminum sheet for closures. The dressing process has significant advantages that include 1) initial machining of the two electrodes into nearly perfect alignment with each other, 2) cleaning of any buildup on the electrode welding face from reaction with the sheet, 3) reshaping the electrode into the correct geometry if the electrode shape was altered by erosion (aluminum welding) or mushrooming (steel welding), and 4) machining a new face into the electrode with a different geometry or different orientation than it originally had. Some previous electrode designs have experienced wear patterns that require extensive reshaping of the worn electrode face. The extensive reshaping requires the removal of a relatively large portion of the welding surface of the electrode and shortens its useful life.
It is a further object of this invention to provide spot welding electrode designs that enable the formation of structural spot welds in highly conductive metal sheets and require less material removal during periodic re-dressing of the electrode thus extending the useful working life of the electrode.